A significant portion of the petrochemical industry is based on the production and utilization of polymerizable monomeric compounds. A wide variety of monomers are useful as precursors of polymeric products including a number of organic monomers which are particularly easy to polymerize. Certain of these readily polymerizable monomers are particularly reactive because of the presence within the monomeric molecule of an activated ethylenic bond, i.e., a non-aromatic carbon-carbon double bond which is activated by conjugation with other multiple bonds between adjacent atoms. Illustrative of polymers produced from such polymerizable monomers are the polystyrenes of the plastics industry and the polyalkadienes and polynitriles of the elastomer industry. The presence of the activated ethylenic unsaturation in the readily polymerizable monomers enables polymerization of the monomers to be conducted under rather mild but carefully controlled conditions and the polymeric products have well established utility.
However, the readily polymerizable monomers often undergo undesirable polymerization, e.g., polymerization prior to the time polymerization is intended, as during the production, purification, storage or handling of the monomers, when exposed to free radicals resulting from the presence of oxygen and other free radical sources. This premature free-radical polymerization can have significant consequences since the polymer formed tends to foul or even plug the mechanical equipment used in the production, purification, handling or even storage of the monomer unless precautions are taken to inhibit or prevent the polymerization. The premature polymerization typically produces polymer product of inferior quality and can be a safety hazard if operating equipment becomes plugged.
To prevent or at least retard the premature free-radical polymerization of readily polymerizable monomers it is conventional to add to the monomer a small but effective amount of an antioxidant or inhibitor. Many if not most of the conventional polymerization inhibitors are complex organic compounds containing functional groups with atoms of oxygen, nitrogen or sulfur. The presence of these atoms in conventional polymerization inhibitors does lead to some disadvantages. The presence of the oxygen, nitrogen or sulfur atoms frequently renders the inhibitor toxic to animal life and can result in the poisoning of polymerization catalysts or molecular sieves used in the subsequent processing and polymerization of the readily polymerizable monomer. It would be of advantage, therefore, to have polymerization inhibitors which have little or no content of the oxygen, nitrogen or sulfur atoms which lead to toxicity or processing problems.